1. Field of the Invention
This invention relates to zoom lenses having soft-focus functions, and, more particularly, to zoom lenses having soft-focus functions with a simple management of moving lens units which constitute part of the zooming section making it possible to introduce aberration into the photographic image while permitting easy control of the variation of the amount of aberration.
2. Description of the Related Art
The prior known methods of softening the photographic image are to attach a soft-focus filter in front of the lens, or to change the axial position of an optical element in the lens system as proposed in Japanese Patent Publication No. SHO 58-1402, or to add an attachment lens as proposed in Japanese Utility Model Publication No. SHO 57-9769. These methods are, however, hardly able to defocus the image uniformly over the entire area of the image format. Another disadvantage of the prior art is the troublesome management in varying the magnitude of the soft-focus effect, because, for example, a series of soft-focus filters have to be interchanged with one another.
In British Patent No. 234,743, Japanese Laid-Open Patent Applications No. SHO 52-76921 and No. SHO 55-52013 (U.S. Pat No. 4,310,221), for mono-focal length lens systems, there has been proposed a method of continuously varying the amount of introduced aberration, for example, spherical aberration, by making variable one of the air separations in the optical system. However, these lens systems are designed for adaptation to special photography and do not work well in photographic situations which may be generally encountered.